The detection of temperature can in general be performed by a single detector, or by an array of such detectors. A single detector may be used in various applications, e.g. as a motion detector. A detector array is used to yield a thermal picture (image) of the observed scene. Such thermal imaging systems are very useful in night vision (e.g., for military use), as driving aids and in heat measurements (e.g. in fire alarm systems). Thermal detectors are implemented using a number of technologies, some of which (e.g., thermocouples) require direct contact with the measured object, and are therefore unsuitable for long distance measuring and imaging. Virtually all of the remote sensing techniques are based on the detection of IR radiation generated by the observed object, and the transformation of this radiation into an electrical signal.
Generally speaking, there are two classes of detectors: The first class may be termed ‘photonic’ detectors. These detectors use the same principle as photodetectors in the visible range, i.e., the photons that are incident upon the detector excite free charge carriers that generate an electrical current. However, due to the low energy of IR photons, these detectors require cooling (typically to 77° K), to suppress the current generated by the thermal excitations within the detector (the “dark” current signal). Details about thermal imaging systems in general, and cooled systems in particular may be found for example in “Handbook of Optics—Fundamentals Techniques and Design”, Michael Bass, Eric W. Van Stryland, David R. Williams, William L. Wolfe (Editors), McGraw Hill 1995, (2nd edition), Vol. 2 Chapters 15–19, which is incorporated herein by reference.
The second class of detectors may be termed thermal energy sensors (TES). Their operating principle is based on sensing the thermal heat generated by the IR radiation emitted by the object and incident upon the detector. A TES converts the IR radiation emitted by the object into heat, and senses the temperature change that this heat causes in the device. A TES is constructed of three elements: (i) means for converting the incident (IR) radiation into heat; (ii) a sensing element of which a certain physical property is very sensitive to temperature changes; and (iii) an apparatus for measuring this property. In principle, a TES does not require cooling for its operation, and can therefore serve as a central element in un-cooled thermal imaging systems. However, it should be noted that a TES is very sensitive to the heat it exchanges with its environment. It is obviously desirable that the small amount of heat produced by the IR radiation absorbed by the TES during one sampling period will generate a maximum change in the temperature of the TES sensing element. Therefore, the TES is constructed to have minimal heat capacity, and to have a much faster thermal response to the heat generated by the absorbed radiation than to the heat that flows into it from its immediate surroundings. A good reference describing TES detectors is “Un-cooled Thermal Imaging: Arrays Systems, and Applications” by Paul W. Kruse, SPIE, 2001, which is incorporated herein by reference
The two most popular implementations of TES are the pyroelectric and the bolometric detectors. The first uses ferroelectric materials, in which the electric polarization is temperature dependent. In some cases, the material is designed to work slightly below the ferroelectric—paraelectric phase transition, where the temperature sensitivity is highest (this is sometimes called the “enhanced pyroelectric effect”). In either the “regular” pyroelectric or the enhanced pyroelectric case, there is a transient current with the change of temperature (due to the change in electrical polarization), which can be measured and used to determine the device temperature.
In the case of bolometric detectors, the physical property that changes with temperature is the resistivity, which is measured with a relatively simple electric circuit. However, since the changes in temperature are quite small, the change in resistivity is difficult to measure. This problem is particularly significant in un-cooled systems.
In summary, there are two classes of thermal imaging systems: (i) cooled systems that are predominantly but not exclusively based on photonic detectors, these systems being in general more expensive, but yielding better performance due to a lower noise level; and (ii) un-cooled systems that are based on thermal energy sensors. Presently known un-cooled systems suffer from low sensitivity and a higher level of noise (which is manifested in a higher value of Noise Equivalent Temperature Difference, NETD), but are considerably cheaper than cooled systems. Both classes of thermal imaging systems are described in the Handbook of Optics and Uncooled Thermal Imaging references above.
As mentioned above, the major drawback of un-cooled TES systems is their relatively high level of noise, which limits their performance. There are several reasons for this relatively high noise. First, the fact that the detector is at high temperature (=room temperature) leads to relatively large fluctuations in its black body radiation. Second, the sampling time in bolometric detectors is quite small due to Joulean heat that develops during the reading process. Third, a chopper is introduced in pyroelectric detectors, which means that about half of the IR radiation is lost. Fourth, the current in pyroelectric detectors is a transient one, and thus the sampling time is limited by the electrical RC time constant. Typical NETD values in both pyroelectric and bolometric techniques are between 50–100 mK. Improvements over the last 20 years have led only to a slight decrease in the NETD.
There is therefore a widely recognized need for, and it would be highly advantageous to have un-cooled thermal detectors with a lower level of noise than existing at present.